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=head1 Logic Programming in Perl

=head2 Introduction

  A programmer who hasn't been exposed to all four of the imperative,
  functional, objective, and logical programming styles has one or more
  conceptual blindspots.  It's like knowing how to boil but not fry.

  Programming is not a skill one develops in five easy lessons.

  -- Tom Christiansen

By now, many Perl programmers know that the language offers support for
imperative, objective, and functional programming.  However, logic programming
seems to be a lost art.  This article attempts to shed a little light on the
subject.  It's not that Prolog can do things that Perl cannot or vice versa.
Instead, Prolog does some things more naturally than Perl -- and vice versa.
In fact, while I introduce you to Prolog, I won't be teaching a bunch of nifty
tricks to make your Perl more powerful.  I can't say what needs you may have
and, in any event, the tools that allow logic programming in Perl are generally
alpha quality, thus making them unsuitable for production environments.

=head2 What is Logic Programming?

Logic programming is somewhat of a mystery to many Perl programmers because,
unlike imperative, objective, and functional styles, Perl does not have direct
support for logic programming.   There is, however, much interest in bringing
logic programming to Perl 6.  With luck the information presented here will not
be merely theoretical.

Logic programming is not as alien as programmers might think.  Regular
expressions, SQL and grammars are all closely related to logic programming.
The shared component of these seemingly disparate technologies is how they
I<describe> their goals rather than state how to achieve it.  This is the
essence of logic programming.  Rather than tell the computer how to achieve a
given goal, we tell the computer what the goal looks like and let it figure out

lib/AI/Prolog/Article.pod  view on Meta::CPAN

 = Goals: 
         male(sally)
 ==> Try:  male(tim) :- null
  <<== Backtrack: 

 [etc.]

Now if you really want to have fun with it, notice how you can rearrange the
clauses in the program at will and Prolog will return the same results (though
the order will likely change).  This is because when one programs in a purely
declarative style, the order of the statements no longer matters.  Subtle bugs
caused by switching two lines of code usually go away.

=head2 Prolog versus Perl

Now that you have a beginning understanding of what Prolog can do and how it
works internally, let's take a look at some of the implications of this.  By
now, you know that the following can be read as "Ovid loves Perl":

 loves(ovid, perl).